Control system for air conditioning units



March 21, 1961 J. A. PIETSCH 2,975,611

CONTROL SYSTEM FOR AIR CONDITIONING UNITS Filed Aug. 31, 1959 F! G. l 22Z4 INVENTOR JOSEPH A. Pl EZTSC H H S ATTORNEY CONTROL SYSTEM FOR AIRCONDITIONING UNITS Joseph A. Pietscll, Louisville, Ky., assignor toGeneral Electric Company, a corporation of New York Filed Aug. 31, 1959,Ser. No. 837,142-

3 Claims. (Cl. 62-151) nite States Patent ditioning units which, in somemanner, direct air from within the enclosure over one heat exchangerduring the cooling season and over the other heat exchanger during theheating season as distinguished from those types of air conditionerswhich reverse the flow of refrigerant through the system to change thefunction of the respective heat exchangers while passing the air fromwithin the enclosure over the same heat exchanger during both thecooling and heating seasons.

In air conditioning units of the type used for supplying heat to anenclosure during the winter or heating season, outdoor air is passedover the evaporator of the unit which operates at temperatures rangingfrom 10 to 3.0 degrees below the temperature of the outdoor ambient.Thus, there are times when the evaporator operates below the freezingpoint of water and, during these times, a layer of frost is deposited onthe evaporator coils. This frost layer acts as a barrier to heattransfer and as the thickness of the layer of frost increases, theefiiciency of air conditioning unit for supplying heat to the .enclosureis greatly reduced. Air conditioners of the type which use a refrigerantreversing valve to interchange the functions of the heat exchangers,i.e., reverse refrigerant flo-w type of air conditioning units, have abuilt-in arrangement for accomplishing the defrosting of the evaporator.That is, the functions of the heat exchangers of these units may bereversed so that the heat exchanger exposed to the outdoor air isoperated as a condenser and becomes warm enough to melt the frost.However, in these reverse refrigerant fiow type of units, during thedefrost cycle, when the refrigerant flow is reversed, the heat exchangerexposed to air from within the enclosure is operated as an evaporatorand takes heat out of the air from within the enclosure to melt thefrost layer. This is likely to cause some discomfort to the occupants ofthe enclosure since the air from the enclosure is cooled during thedefrosting periods. Since air valve type air conditioning units are notsupplied with a means for reversing the refrigerant flow within thesystem they have no builtin arrangement for utilizing hot gas fordefrosting the evaporator.

It is, therefore, an object of the present invention to provide anarrangement wherein hot gas defrosting of the evaporator is provided inan air valve type air conditioner.

It is another object of the present invention to provide an air valvetype air conditioner having a control arrangement employing hot gasdefrost for the evaporator which operates quickly and efficientlywithout removing heat to any substantial degree from the enclosure and,therefore, causes no discomfort to the occupants of the conditionedenclosure.

It is a further object of the present invention to provide an improvedcontrol system for an air conditioning unit of the air valve type whichis adapted to heat and cool an enclosure.

Further objects and advantages of the invention will become apparent asthe following description proceeds, and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

In accordance with the present invention, there is provided an airconditioning unit having the usual refrigeration system including anevaporator, a condenser, and a compressor connected in refrigerant flowrelationship. The system is provided with a refrigerant expansion meansbetween the evaporator and the condenser for producing a pressure dropin the system between these two members. Also connected between theevaporator and the condenser is a normally closed bypass conduit fordirecting refrigerant around the expansion means directly into theevaporator from the condenser. The unit is provided with fans or airmoving means which, during the heating season, are adapted to circulateseparate streams of air from the inside and from the outdoors over thecondenser and the evaporator respectively. In order to automaticallydefrost the evaporator, there is provided a pressurestat which isresponsive to the air pressure drop across the evaporator due to frostaccumulation thereon. The pressurestat opens a valve closing therefrigerant bypass passage whenever a predetermined air pressure dropoccurs across the evaporator thereby shunting refrigerant from thecondenser around-the expansion means directly into the evaporator. Thepressurestat also energizes a holding relay which, in turn, interruptsthe power to the fan motor and stops the operation in the fans. Theholding relay also maintains the valve in the bypass conduit open whenthe air ceases to flow through the evaporator and a pressure drop is nolonger present across the evaporator to energize the pressurestat. Adefrost terminating switch, responsive to the temperature of refrigerantflowing from the evaporator through the suction line, de-energizes theholding relay so that the valve in the bypass conduit is closed andpower is again supplied to the fan motor to resume normal operation ofthe unit.

As a further aspect of the invention the control system includes aselector control means for conditioning the thermostat of the unit tooperate the refrigeration system according to increases in the enclosureair temperature above a predetermined temperature or according todecreases in the enclosure air temperature below a predetermined airtemperature. The selector control also automatically adjusts theoperation of the fan motor to a low speed whenever the thermostat isconditioned to operate the refrigeration system according to decreasesin enclosure air temperature below a predetermined temperature.

For a better understanding of the invention reference may be had to theaccompanying drawing in which:

Fig. 1 is a schematic diagram of a self-contained air conditioning unitincorporating the present invention; and

Fig. 2 is a line diagram illustrating the components of the airconditioner and an electrical circuit for the control system of thepresent invention.

Referring now to Fig. 1, there is shown an air conditioner of theself-contained type including a casing 2 adapted to be mounted in anaperture or Window in the outer wall of an enclosure with one side ofthe casing facing the enclosure 3 and the other side of the casingexposed to the outdoors 4. The casing is divided by a barrier into twoseparate compartments hereinafter designated the condenser compartment 7and the evaporator compartment 8. Within the casing is a refrigerationsystern including a condenser 9, an evaporator 11 and a compressor 12connected by suitable refrigerant conducting tubing in refrigerant flowrelationship. The evaporator 11 is positioned in the evaporatorcompartment 8 while the condenser 9 and the compressor 12 are bothmounted in the condenser compartment 7. Suitable expansion means such asthe capillary 10 or an expansion valve are provided between thecondenser 9' and the evaporator 11 for producing a pressure drop in therefrigeration system between these two members. An accumulator isprovided in the suction conduit between the evaporator 11 and thecompressor 12 for collecting liquid refrigerant which, under certainconditions of operation, overflows the evaporator. The accumulator 15stores liquid refrigerant and prevents it from flowing directly into acompressor.

The air conditioner of the present invention is of the air valve type.That is, during operation, the evaporator always removes heat from theair within the evaporator compartment 8 while the condenser 9 dischargesheat or is cooled by the air within the condenser compartment 7. As maybe seen in Fig. 1, the evaporator compartment is divided into air inletand outlet sections 13 and 14 respectively by the fan shroud 16 and afan 17 which also circulates air through the evaporator compartment. Adamper means or closure panel 18, which is movable to either theenclosure or outdoor facing sides (as indicated by the dotted lines inFig. 1) of the evaporator compartment, makes it possible to circulateeither outdoor or enclosure air through the evaporator compartment whereit is cooled by the evaporator. Similarly, the condenser compartment isdivided by the shroud 19 and the fan 21 into condenser compartment inletand outlet sections 22 and 23. A damper means or closure panel 24,similar to the panel 18 in the evaporator compartment, is arranged todirect air from the outdoors or from the enclosure into the inletsection 22 merely by moving the panel 24 from one side of the condensercompartment to the other. The fans 17 and 21 are both driven by a singlefan motor which, for reasons to be hereinafter explained, is preferablya two speed motor.

With the damper means or closure panels 18 and 24 positioned as shown inFig. 1, the air conditioner is adapted to heat the enclosure 3. Morespecifically, air is circulated through the condenser compartment 7 fromthe enclosure where it is heated by the condenser and discharged backinto the enclosure as indicated by the arrows in Fig. 1. Outdoor air isdrawn from the outdoors into the evaporator compartment inlet section 13where it encounters the evaporator 11 and gives up heat to theevaporator. The outdoor air, after giving up heat to the evaporator, isthen discharged tothe outdoors through the outlet section 14. The heatremoved from the outdoor air by the evaporator 11 is carried through thesystem by the refrigerant and gives up this heat at the condenser toheat the air from the enclosure.

Movement of the damper means or closure panels 18 and 24 into theirrespective positions on opposite sides of the case may be accomplishedautomatically or manually. It is assumed in the following descriptionthat the occupants of the enclosure move the panels to their properpositions on the case whenever they desire to change the operation ofthe unit from cooling to heating or vice versa. In order to provideadditional heat for the enclosure whenever the refrigeration system ofthe unit is not capable of maintaining the desired temperature withinthe enclosure there is provided an auxiliary or supplementary heater 25in the form of a resistance heater. The heater 25 is positioned in thecondenser compartment and heats the air circulated therethrough as itenters the enclosure.

The outdoor air, in passing over the evaporator 11, is cooled and, if ofsufficient moisture content, it deposits a certain amount of moisture onthe coils and evaporator surfacesf Under certain conditions whichnormally occur when the outdoor temperature is above a certaintemperature, such as 42 F., the evaporator operates at a temperatureabove the freezing point of water. Under these conditions, the waterdeposited on the evaporator by the outdoor air drops from the evaporatorinto a suitable collection tray or sump 26 in the bottom of theevaporator compartment. This water is removed from the unit through thedrain 27 and suitable drain conduits (not shown) which lead to theoutdoors or to any other suitable drain area. Whenever the outdoortemperature drops too low, such as below the temperature 42 F., theoperating parameters of the refrigeration system cause the evaporatortemperature to drop below 32 F. or below the the freezing point ofwater. Under these latter conditions of operation, there is a build-upof frost on the evaporator 11 if the outdoor air is cooled below the dewpoint. This frost barrier acts as an insulation and prevents effectiveheat transfer between the evaporator coils and the air within theevaporator compartment, thus, greatly decreasing the amount of heatremoved from this air which, in turn, decreases the heat output at thecondenser. As will hereinafter be explained the present invention dealswith a control arrangement for controlling the operation of the airconditioning unit to provide either heating or cooling for the enclosureand for automatically defrosting the evaporator whenever the frostbuild-up thereon is such as to impair the efliciency of the unit.

Referring now to Fig. 2, there is shown a control arrangement forselectively operating the above-described air conditioner on the coolingor the heating cycle and for automatically defrosting the evaporator ofthe unit whenever there is an excessive frost build-up thereon. As maybe seen in Fig. 2, power is supplied to the air conditioner throughlines 31 and 32 and the main switch 33. A selector control means 34,which is manually operated by the occupants of the enclosure, is adaptedto condition the unit for operation on the heating or cooling cycle. Theselector control means includes a pair of switches 36 and 37 which aremovable to selective positions for energizing, or partially energizing,various circuits leading to the components of the air conditioner. Thus,switch 36, which will hereinafter be referred to as the comfort controlswitch, is movable to complete the circuit between contacts 33 orcontacts 39 while switch 37, which will hereinafter be referred to asthe fan speed switch, is movable to separate positions to complete thecircuit between contacts 41.

A thermostat 43 is provided which is conditioned by the selector controlmeans to operate the refrigeration system of the air conditioneraccording to finite tem perature increases or decreases above or below apredetermined temperature set on the thermostat by the occupants of theenclosure. Normally, the thermostat 43 with its temperature sensingmeans is placed on the unit in a position where it can be exposed to theair from the enclosure during both the heating and cooling seasons. Thisis schematically illustrated in Figs. 1 and 2 by the thermal bulb 43aconnecting with the thermostat 43. The thermal bulb 43a operates anexpansion bellows 43b, or other responsive means, in the thermo statwhich, in turn, energizes mechanical linkage connecting with electricalswitches in the thermostat. When the contacts 38 of the comfort controlswitch 36 are closed, current is supplied to the thermostat through theheating cycle contact 44. With the current supplied to the heating cyclecontact 44, the temperature sensing member 43a, through bellows 43b ofthe thermostat, then energizes the circuits leading to the compressorand the other components of the system by closing the switch 46 wheneverthe temperature of the enclosure falls below a predeterminedtemperature.

Conversely, whenever the comfort control switch 36 is positioned acrosscontacts 39 so that current is supplied to contact 47, or the coolingcycle contact of the thermostat, then the circuits leading to thecompressor and to the other components of the unit are energized by theswitch 46 which is forced to move across contact 47 by the temperaturesensing member 43a, through bellows 43b of the thermostat whenever thetemperature of the enclosure rises above a predetermined temperature.

It will be noted that the thermostat 43 is also provided with a heaterswitch 48. The heater 25 is energized by the heater switch 48 which isautomatically moved by the temperature responsive device of thethermostat across contact 49 whenever the room temperature drops acertain number of degrees below that set by the occupant. That is, forexample, if the thermostat were set at 75 F. and the actual roomtemperature fell to 70 F., it would be anindication that the heat loadof the room was more than could be met by the refrigeration systemalone, and the supplementary heater 25 would then be energized to supplyadditional heat. Thermostats having the abovedescribed type of operationare well known in the art and a detailed description of their structureis not believed necessary in this specification.

Operation of the fan speed switch 37 is coordinated with the operationof the comfort control switch 36 so that the fan motor is operated onlow speed only when the comfort control switch 36 is positioned tooperate the refrigeration system in a manner to supply heat to theenclosure. That is, whenever the air conditioner is to be operated tosupply heat to the enclosure or, more specifically, whenever therefrigeration system is operated upon the occurrence of temperaturedecreases within the enclosure, the fans are operated at low speed. Thecondenser fan 21 of air conditioning units is normally much larger thanthe evaporator fan 17 and its noise is correspondingly greater. Sincethe condenser fan 21 is exposed to the inside during heating, it isdesirable, in order to reduce the noise level of the unit during thisoperation, to operate the fans at a lower speed. When the comfortcontrol switch 36 is in the heating position across contacts 38, the fanspeed switch is moved across contacts 41. Thus, during the heatingoperation, current is supplied to the low speed terminal 51 of the fanmotor 20 through the circuit including electrical conduits 52, 53, 54,55 and 56. During the cooling operation when the comfort control switch36 is bridged across contacts 39, the fan speed switch closes contacts42 and current is then supplied to the high speed terminal 57 of the fanmotor through the electrical conduit 58.

As stated previously, the frost build-up on the evaporator 11 during theheating cycle reduces the efficiency of the system to supply heat at thecondenser. In order to melt the frost on the evaporator whenever thebuild-up thereon creates too great a pressure drop across theevaporator, a bypass passage 59 is connected between the condenser 9 andthe evaporator 11 for shunting warm refrigerant or hot gas from thecondenser directly into the evaporator. During normal operation of thesystem, the bypass conduit 59 is closed but means are provided in theform of a pressurestat control 61 and valve 62 for opening the conduitwhenever a predetermined pressure drop occurs across the evaporator. Inthe illustrated embodiment of the invention the pressurestat 61 isprovided with a first static tube 61b for sampling air pressure on theupstream side of the heat exchanger 11 and a second static tube 610 forsampling air pressure on the downstream side of the heat exchanger 11.Pressurestats of this type are well known in the art and a more detaileddescription thereof is not believed necessary for a full understandingof the invention except insofar as to state that the instrument measuresthe difference in pressures as sensed by the two static tubes and isprovided with means for actuating a switch in accordance with apredetermined pressure differential across the heat exchanger. Thepressurestat 61 is adapted to close the normally open switch 61a (see inFig. 2) whenever the pressure drop across the evapflow of air throughthe evaporator becomes more and p more restricted, the pressure dropacross the evaporator gives an indication of the frost build-up on theevaporator. As may be seen in Fig. 2, when the pressurestat switch 61ais closed to its dotted line position in Fig. 2 by the pressurestat,electrical current is supplied to the bypass valve solenoid coil 62awhich then causes the valve 62 (of Fig. 1) to move to the open position.In this manner warm liquid refrigerant or vaporous refrigerant is passedor shunted around the capillary 10 directly into the evaporator. Sincethe refrigerant flows generally in the same direction through thesystem, there is no need to shut off the compressor When the valve 62 isopened.

During the defrost cycle, circulation of cold outside air over theevaporator 11 is undesirable. In order to stop the fan motor 20 duringthe defrost cycle, a relay 63 is provided, which contains a coil 63aalso in series with the pressurestat switch, that is energized when thepressurestat switch 61a is closed. The relay coil 63a operates a relayswitch 63b which interrupts the current flowing to the fan motor. Morespecifically, relay switch 63b which is normally positioned across thecontact 66, is upon energization of the relay coil 63a, moved away fromcontact 66 into the dotted line position shown in Fig. 2 to open thecircuit between electrical conduits 54 and 55 thereby interrupting thecurrent flow to the fan motor 29. When the evaporator fan 17 ceases torotate there is no longer a pressure drop across the evaporator, and thepressurestat 61 will open the pressurestat switch 61a. Since the openingof the pressurestat switch would ordinarily interrupt the power suppliedto both the bypass valve solenoid 62a and the relay coil 63a, means mustbe provided for supplying electrical current to these members when theevaporator fan stops rotating so that the defrost operation will becontinued. This is accomplished by causing the relay switch 63b to moveacross contact 67 (into the dotted line position shown in Fig. 2)thereby shunting current around the pressurestat switch 61a through theconduits 54 and 68. Thus, even though the pressurestat switch has movedto its normally open position, the relay switch 63b still interrupts thecurrent to the fan motor and completes the holding circuit to the bypassvalve solenoid 6211 as well as to the holding relay coil 63a. The hotgas from the condenser is, thereby, continuously delivered through thebypass conduit 59 to the evaporator 11 for melting the frost on theevaporator.

In order to terminate the defrost operation, means responsive to thetemperature of the refrigerant leaving the evaporator are provided forinterrupting the electrical current to the holding relay 63a. In Fig. 1there is shown a temperature responsive bulb 69 which actuates anexpansion device 69a when the temperature of the refrigerant in thesuction conduct 70 exceeds a predetermined value. The bulb and expansiondevice opens the defrost terminating switch 69b thereby de-energizingthe holding relay coil 63a. The defrost terminating switch 6% is in theopen or dotted line position of Fig. 2 whenever the temperature of therefrigerant is above a predetermined temperature such as 37 F. Thus, thecircuit to the bypass valve solenoid 62a can never be energized unlessthe temperature of the refrigerant leaving the evaporator issufficiently cold to close switch 6%.

The above-described invention provides a simple control arrangement foran air conditioner of the air valve type. It also proposes a defrostarrangement adapted to operate quickly and effectively to defrost theevaporator without utilizing the heat of the enclosure, to anysubstantial extent, to provide the heat necessary to melt the frost.

While, in accordance with the patent statutes, there has been shown anddescribed what at present is considered to be the preferred embodimentof the invention, it will be obvious to those skilled in the art thatvarious changes and modifications may be made therein without depart- Zing from the invention and it is, therefore, the aim of the appendedclaims to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An air conditioner for both heating and cooling an enclosurecomprising a casing adapted for mounting in an outer wall of saidenclosure, a refrigeration system in said casing including a compressor,a condenser and an evaporator connected in refrigerant flowrelationship, refrigerant expansion means connected between said condenser and said evaporator for expanding refrigerant from condenserpressure to evaporator pressure, a normally closed bypass conduit alsoconnected between said condenser and said evaporator for bypassingrefrigerant from said condenser around said expansion means directlyinto said evaporator, fans in said casing for circulating separate airstreams over said condenser and said evaporator respectively, anelectrical motor for driving said fans, openings in said casingcommunicating with said enclosure and the outside, damper means in saidcasing for directing air from inside and outside said enclosure oversaid condenser and evaporator respectively, means for defrosting saidevaporator including a pressurestat responsive to the air pressure dropacross said evaporator due to frost accumulation thereon, means actuatedby said pressurestat for opening said bypass conduit to pass refrigerantaround said refrigerant expansion means from said condenser directlyinto said evaporator, a holding relay also energized by saidpressurestat for interrupting the power to said fan motor and formaintaining said bypass conduit open when said pressurestat becomesinoperative as a result of the discontinued air flow through saidevaporator, and means responsive to the temperature of refrigerantleaving said evaporator for deenergizing said holding relay so that saidbypass conduit is again closed and power is again supplied to said fanmotor for circulating air through said evaporator and said condenserrespectively.

2. An air conditioner for both heating and cooling an enclosurecomprising a casing adapted for mounting in an outer wall of saidenclosure, a refrigeration system in said casing including a compressor,a condenser and an evaporator connected 'in refrigerant flowrelationship, refrigerant expansion means connected between saidcondenser and said evaporator for expanding refrigerant from condenserpressure to evaporator pressure, 9. normally closed bypass conduit alsoconnected between said condenser and said evaporator for bypassing saidrefrigerant from said condenser around said expansion means directlyinto said evaporator, fans in said casing for circulating separate airstreams over said condenser and said evaporator respectively, a high andlow speed motor for driving said fans, openings in said casingcommunicating with said enclosure and the outside, damper means in saidcasing for directing air from inside and outside said enclosure oversaid condenser and evaporator respectively, a thermostat adapted tosense the temperature of the air within said enclosure, a selectorcontrol means for conditioning said thermostat to operate saidrefrigeration system according to increases in air temperature Withinsaid enclosure or according to decreases in air temperature within saidenclosure, said selector control means also switching said fan motor tooperate on low speed only when said thermostat is conditioned to operatesaid refrigeration system according to decreases in the air temperaturewithin said enclosure, means for defrosting said evaporator including apressurestat responsive to the air pressure drop across said evaporatordue to frost accumulation thereon, means actuated by said pressurestatfor opening said bypass conduit to pass refrigerant around saidrefrigerant expansion means from said condenser directly into saidevaporator, a holding CJI relay also energized by said pressurestat, aswitch means energized by said holding relay for interrupting the powerto said fan motor and for maintaining said bypass conduit open when saidpressurestat becomes inoperative as a result of the discontinued airflow through said evaporator, and means responsive to the temperature ofre frigerant leaving said evaporator for de-energizing said holdingrelay so that said bypass conduit is again closed and power is againsupplied to said fan motor for circulating air through said evaporatorand said condenser respectively.

3. An air conditioning unit for both heating and cool-' ing an enclosurecomprising a casing adapted for mounting in an outer wall of saidenclosure, a refrigeration system in said casing including a compressor,a condenser, and an evaporator connected in refrigerant flowrelationship, refrigerant expansion means connected between saidcondenser and said evaporator for expanding refrigerant from condenserpressure to evaporator pressure, a normally closed bypass conduit alsoconnected between said condenser and said evaporator, a solenoidoperated valve for opening said bypass conduit to bypass refrigerantfrom said condenser around said expansion means directly into saidevaporator, fans in said casing for circulating separate air streamsover said condenser and said evaporator respectively, a high and lowspeed motor for driving said fans, openings in said casing communicatingwith said enclosure and the outside, damper means in said casing fordirecting air from inside and outside said enclosure over said condenserand evaporator respectively, control means for said air conditioningunit comprising a thermostat exposed to air from said enclosure, aselector control having first and second switches, said first switchadapted to condition said thermostat to operate said refrigerationsystem according to increases in air temperature within said enclosureor according to decreases in air temperature within said enclosure, saidsecond switch adapted to condit-ion said fan motor for operation oneither high or low speeds, said second switch automatically connectingsaid fan motor to low speed operation only when said first switchconditions said thermostat to operate said refrigeration systemaccording to decreases in air temperature within said enclosure, meansfor defrosting said evaporator including a pressurestat switchresponsive to the air pressure drop across the evaporator due to frostaccumulation thereon, said pressurestat switch being connected in serieswith said solenoid of said bypass valve for opening said bypass valve topass refrigerant around said expansion means directly from saidcondenser into said evaporator to warm said evaporator, a holding relayalso connected in series with said pressurestat switch, a faninterrupting switch energized by said holding relay for stopping saidfan motor, a holding circuit completed by said fan interrupting switchfor supplying power to said holding relay and said solenoid of saidbypass valve for operating said bypass valve when said pressurestatbecomes inoperative as a result of the discontinued air flow throughsaid evaporator, and a defrost terminating thermostat responsive totemperature or refrigerant leaving said evaporator, said defrostterminating thermostat including switching means in said holding circuitfor de-energizing said holding relay so that said bypass conduit isagain closed and power is again supplied to said fan motor forcirculating air through said evaporator and said condenser respectively.

References Cited in the file of this patent UNITED STATES PATENTS2,001,028 Kitzmiller May 14, 1935 2,667,757 Shoemaker Feb. 2, 19542,728,197 Ellenberger Dec. 27, 1955 2,911,797 Stocking Nov. 10, 1959

